I remember that your switch is on the dryer, correct? If so, that is far away from the compressor, and since the switch is on the highside circuit, 115 is not near enough to trigger it. You probably can't add enough pressure statically, so are going to have to jumper the switch with a simple wire.

Jumper the switch, add some refrigerant with the compressor pumping (just a little - a minute or two), stop and put the switch back in play and see if it is now triggered. Repeat as necessary.

PS - you can verify the switch is triggered or not by testing it for continuity - don't need to keep putting the socket on and off, in other words.